1. Field of the Invention
The present invention relates to a subsurface fluid distribution system, and more particularly, to an underground array of interconnected leaching chambers that enable the unpressurized, subsurface dispersion of sewage and irrigation waters.
2. Description of the Prior Art
The underground treatment and dispersal of sewage effluent, as well as subsurface irrigation systems, require enhanced rates of water flow for maximum efficiency. Such flow rates can be achieved by either reliance on system pressurization or by attempting to enhance the efficiency of the more "natural" (and lower energy) processes such as diffusion, capillary action, and root-system absorption.
Turning first to sewage treatment, after initial processing within a septic tank, an effluent solution is discharged that includes partially-dissolved solids. Further bacterial action results in substantially complete dissolution of the solids, permitting its dispersal, with the liquid water, as the latter is absorbed into the surrounding soil. Most commonly this secondary treatment and dispersion is accomplished by passage of the effluent through a "leach line"--perforated piping placed within stone-filled trenches, and covered by a thin layer of top soil.
As a result of good initial reliability, plus statutory "encouragement" through sanitation regulations, the design specifications for leach lines has changed very little over the past sixty years, apart from occasional variations in physical dimensions and structural components. A trench is dug, and then partly filled with stone. A perforated pipe is laid on top of the stone, which is then covered by additional stone. A semi-permeable membrane such as building paper or a layer of straw is placed over the stones, followed by a topping layer of soil.
A typical leach line is twenty-four inches in width and eighteen inches in depth. While seemingly quite shallow, at eighteen inches sufficient oxygen is present in the soil to support aerobic microorganism decomposition. Such organisms and processes are considerably more efficient in breaking down the semi-solid sewage materials than is the decomposition that occurs in a deeper, anaerobic (no oxygen) environment. In addition, most soils are also less permeable to water as the soil depth increases. The increasingly compressed soil structure at greater soil depths results in slower rates of water flow through the soil, and less efficient dispersal of the sewage effluent along the leach line.
In addition to providing support for the distribution pipe, the stone fill in the leaching trenches increases the effective decomposition surface area and creates effluent storage volume within the trench. At three-quarter inch to one-and-a-half inches in diameter, Number "2" stone provides a sufficient number and size of void space to efficiently retain and distribute the sewage effluent throughout the leach line. One cubic foot of No. 2 stone provides a maximum of 3.25 gallons or 43% of the available storage volume for the sewage effluent. For a typical twenty-four inch wide by fifty-foot long leach line, this results in approximately 6.5 gallons per foot, or 325 gallons of effluent storage volume. In addition to providing capacity for surges in liquid flow, this pore or void volume also enhances the effective area for fluid dispersion at the soil-stone interface.
As initially installed, the effluent discharge surface area can be considered to be the "wetted" area of the sides and bottom of the stone-soil interface. At a preferred width of twenty-four inches, the resultant stone-soil interface consists of the bottom, twenty-four inch width and that lower "wetted" portion of the 2, twelve-inch side walls. Since the side walls provide equivalent absorbency to that of the bottom area, the fifty-fifty proportion of soil contact area, as between the twenty-four inch bottom width and the 2, twelve-inch side walls, maximizes the absorption efficiency between the leach line volume and the available soil contact area. Additionally, as the bottom stone layer becomes plugged (for reasons discussed below), an additional side wall surface area becomes available for liquid absorption as the liquid level rises in the leach line.
The final layer of the leach line consists of the top soil cover. A six-inch depth of soil represents a satisfactory compromise between providing pathogen-to-atmosphere separation as well as providing an optimal soil matrix for grass development. While a deeper top soil layer would beneficially increase the atmosphere-to-sewage separation distance, it would detrimentally decrease the evaporative loss rate from the leach line as well as result in the formation of an unstable layer of soil immediately above the semi-permeable membrane. Lying too deep to be stabilized by the grass roots, this layer of soil would tend to gradually silt into the stone-filled layer below, increasing an erosion problem that is inherent in stone-layer leach lines.
In addition to its often being costly to obtain and labor-intensive to install, stone fill is not a stable component of subsurface soil structure. The voids within the stone fill, while beneficial for liquid storage, are subject to a slow siltation process, whereby the surrounding soil gradually invades and fills these interstices. This slow collapse of the trench walls and ceiling into the stone voids reduces both the available pore space as well as the effective effluent discharge surface area. Further, this siltation process itself establishes subsurface liquid flow channels that tend to accelerate the siltation process. As one example, the gradual collapse of the trench walls and overlying soil of the original leach line creates a depression along the length of the leach line. Once formed, an increased volume and flow of surface rainwater is channeled into this depression, further accelerating the siltation of the surrounding soil into the stone bed.
In addition to siltation, the traditional stone-fill leach lines present other inherent steady-state operational problems. Typically, the perforated leach line pipe is four-inch (diameter) PVC pipe that passively delivers sewage effluent to the stone fill. The void spaces permit liquid sewage to flow throughout the field and, with no capillary action within the stone fill, the liquid sewage settles over time to occupy the lowest portions of the leach line.
Additional sewage effluent flowing into the field will then accumulate on top of the initial fluid flows. The passive stone fill leach line provides no active mechanism to either evenly distribute this sewage (horizontally) throughout the field or to draw the water upward (vertically), and thereby permit its interface with the upper soil portions of the trench. Over time, stagnation results and the lowest portions of the leach line trench become anaerobic. This further slows the decomposition of organic material in the lower sewage layer, as well as results in the production of incomplete decomposition by products (sludge) that plug up additional soil interface areas. With the lower portions of the trench no longer able to accept additional sewage flows, adding additional effluent only results in an increasingly deep anaerobic stagnant layer. The leach line slowly "dies".
Although ultimately susceptible to these siltation and/or stagnation processes, initially the leach line offers effective pathogen containment and waste water disposal by three separate mechanisms: (1) A reserve fluid storage capacity is provided by the stone voids; (2) The soil-stone interface establishes a mechanism for fluid absorption in the surrounding soil; and (3) Evapo-transpiration fluid loss is provided through the shallow construction and the grass root absorption.
There thus remains a need for a subsurface fluid distribution system having a reserve fluid storage capacity that, over time, maintains both fluid access to an active fluid/soil interface, for absorption into the surrounding soil, as well as an enhanced evapo-transpiration loss through the active absorption of the fluid by plant roots and a system that minimizes the negative attributes of siltation and stagnation.
In terms of optimized crop production and healthy plant growth, the critical factor for irrigation is the efficiency by which water is provided to the root zone area of the soil. For above-ground watering systems, before any benefit is realized by the growing plants the irrigation water must first enter the soil and then penetrate to the root zone. Wetting only the above-ground portions of the plant, or the layers of organic material and soil above the root zone is, at best, of no value. In fact, in many areas of the west such practices are harmful, in terms of mineral salt buildup.
Light, above-ground waterings encourage shallow rooting, resulting in plants that are less able to withstand periods without water. On the other end of the scale, heavy watering will frequently result in excessive amounts of water lost through evaporation and water run-off. Additionally, to avoid increased instances of plant disease, the timing of the watering is crucial, to avoid creating excessive above-ground plant moisture during the nighttime hours.
In an effort to avoid many of the forgoing problems, as well as the many maintenance problems associated with above-ground watering systems, underground irrigation has become increasingly popular. In theory such irrigation systems place the water almost directly into the root zone and eliminate water losses due to evaporation. Additionally, with the irrigation system substantially located below ground, all of the distribution pipes and delivery systems are protected from most forms of damage due to the movements of people and machinery.
Unfortunately, this subterranean location also makes it difficult to recognize when a problem in a system has first developed. Delivery heads or ports can become plugged, distribution lines break, and as a result, interrupt the delivery of water to the root zone, to the detriment of plant growth. Additionally, since most underground irrigation systems make use of pressurized and filtered water, breaks in the distribution lines can rapidly create severe subsurface erosion problems, as well as unwanted and sudden water losses. There thus exists a need for underground irrigation systems that rely on a passive water flow to distribute water evenly and consistently to the root zone areas of cultivated crops and turf grasses.